This application claims Paris Convention priority of DE 100 41 672.1 filed Aug. 24, 2000 the complete disclosure of which is hereby incorporated by reference.
The invention concerns a magnet arrangement for generating a magnetic field in the direction of a z axis in a working volume disposed about z=0, with a magnet coil system having at least one current-carrying superconducting magnet coil, and with one further current-carrying coil system which can be fed via an external current source to produce a magnetic field in the working volume which is substantially different from zero, in particular a magnetic field of an amount  greater than 0.2 millitesla per ampere current, and optionally with one or more additional superconductingly closed current paths, wherein the magnetic fields in the z direction produced by induced currents through the additional current paths during operation and the field of the current-carrying coil system in the working volume do not exceed 0.1 Tesla.
A magnet arrangement of this type comprising a superconducting magnet coil system and a further coil system fed via an external current source, however, without additional superconductingly closed current paths, is known e.g. from the EPR (Electron Paramagnetic Resonance) system ELEXSYS E 600/680, distributed since 1996 by the company Bruker Analytik GmbH, Silberstreifen, D-76287 Rheinstetten (company leaflet).
Superconducting magnets are used for different applications, in particular, different magnetic resonance methods. Some of these methods require modulation of the field strength in the working volume during an experiment. In particular, the use of a superconducting magnet has considerable disadvantages if the field modulation is produced through variation of the current in the main coil system. The main coil system typically has a high self-inductance and therefore permits only slow current and field changes.
Connection of current feed lines from the room temperature region to the cooled superconducting magnet during operation disadvantageously affects the cooling of the superconducting magnet coil system. If the region within which the magnetic field strength is to be modulated is not too large (in particular smaller than 0.1 Tesla), field modulation can be produced through varying the current in a coil system which supplements the main coil system.
A further field of use of field-generating additional coils in a superconducting magnet system are so-called superconducting Z0 shim devices. A current change in such a device compensates for a drift in the main coil system over a certain period of time, without having to reset the current in the main coil.
The main focus of the invention is the dimensioning of magnet arrangements having an additional current-carrying coil system which can be fed via an external current source to produce a magnetic field in the working volume which is substantially different from zero, in particular, the dimensioning of magnet arrangements having a superconducting magnet with active stray field compensation and further superconducting current paths.
An additional field-producing coil system in a magnet arrangement must produce a relatively strong field while occupying as little space as possible. To achieve the required field strengths, an additional field-producing coil system must frequently be disposed close to the working volume of the magnet arrangement. This produces undesired xe2x80x9cexpansionxe2x80x9d of the superconducting coil system and associated increased costs.
In contrast thereto, it is the underlying purpose of the present invention to modify a magnet arrangement of the above-mentioned type with as simple means as possible such that an additional field-producing coil system can be integrated in the magnet arrangement which xe2x80x9cexpandsxe2x80x9d the main coil system to a lesser extent while nevertheless maintaining the required functions.
This object is achieved in accordance with the invention in that the efficiency of the additional field-generating coil system is improved by utilizing the interaction between the additional field-generating coil system and the remaining magnet arrangement to produce the field. In addition to inductive couplings between the superconducting magnet coil system and further superconductingly closed current paths, an arrangement in accordance with the invention also uses the diamagnetic behavior of the superconducting material in the superconducting magnet coil system, which is characterized in that field changes of less than 0.1 Tesla, which occur e.g. during charging of an additional field-generating coil system, are expelled from the superconducting volume portion of the magnet coil system.
This manifests itself in a redistribution of the magnetic flux of the field changes in the magnet arrangement which effects the reaction of the superconducting magnet coil system and the additional superconductingly closed current paths to a current change in the additional field-generating coil system, since this reaction is determined by the principle of conservation of the magnetic flux through a closed superconducting loop. The present invention utilizes the interaction between the additional field-generating coil system and the residual magnet arrangement for generating a field such that the variable gDeff=gDxe2x88x92gTxc2x7(Lclxe2x88x92xcex1Lcor)xe2x88x921xc2x7(L←Dclxe2x88x92xcex1L←Dcor) is calculated and the magnet arrangement is optimized such that |gDeff| greater than 1.2xc2x7|gDeff,cl|, wherein
xe2x80x83gDeff,cl=gDxe2x88x92gTxc2x7(Lcl)xe2x88x921xc2x7L←Dcl.
These variables have the following definitions:
gDeff: Field contribution per ampere current of the additional field-generating coil system in the working volume taking into consideration the field contributions of the additional field-generating coil system itself and the field change due to currents induced in the superconducting magnet coil system and additional superconductingly closed current paths during charging of the additional field-generating coil system and taking into consideration the diamagnetic expulsion of small field changes from the volume of the magnet coil system,
gDeff,cl: Field contribution per ampere current of the additional field-generating coil system in the working volume taking into consideration the field contributions of the additional field-generating coil system itself and the field change due to currents induced in the superconducting magnet coil system and in additional superconductingly closed current paths during charging of the additional field-generating coil system while neglecting the diamagnetic expulsion of small field changes from the volume of the magnet coil system,
xe2x88x92xcex1: average magnetic susceptibility in the volume of the magnet coil system with respect to field changes which do not exceed the amount of 0.1 T, wherein 0 less than xcex1xe2x89xa61,
gT=(gM,gP1, . . . ,gPj, . . . ,gPn),
gPj: Field per ampere of the current path Pj in the working volume without the field contributions of the current paths Pi for ixe2x89xa0j, which react inductively to flux changes, and the magnet coil system,
gM: Field per ampere of the magnet coil system in the working volume without the field contributions of additional current paths which inductively react to flux changes,
gD: Field per ampere of the additional field-generating coil system in the working volume without the field contributions of additional current paths, which react inductively to flux changes, and of the magnet coil system,
Lcl: Matrix of the inductive couplings between the magnet coil system and additional current paths which react inductively to flux changes, and among these additional current paths,
Lcor: Correction for the inductance matrix Lcl, which would result with complete diamagnetic expulsion of disturbing fields from the volume of the magnet coil system,
L←Dcl: Vector of inductive couplings of the additional field-generating coil system with the magnet coil system and the additional current paths which react inductively to flux changes,
L←Dcor: Correction for the coupling vector L←Dcl, which would result with complete diamagnetic expulsion of disturbance fields from the volume of the magnet coil system.
In a preferred embodiment of the inventive magnet arrangement, the magnet arrangement is part of an apparatus for nuclear magnetic resonance spectroscopy, e.g. for EPR or NMR. Such apparatus require frequent modulation of the magnetic field in the working volume to sweep the resonance line in a so-called field sweep. This is usually effected with an additional coil system which supplements the magnet coil system and can be dimensioned particularly effectively in an arrangement in accordance with the invention.
One embodiment of the inventive magnet arrangement is particularly advantageous, wherein the superconducting magnet coil system comprises a radially inner and a radially outer coaxial coil system which are electrically connected in series, wherein these two coil systems each generate one magnet field in the working volume of opposing direction along the z axis. In such an arrangement, the magnetic shielding behavior of the superconductor in the magnet coil system typically has a particularly strong effect on the effective field strength gDeff of certain additional field-generating coil systems in the working volume.
In a further development of this embodiment, the radially inner coil system and the radially outer coil system have dipole moments approximately equal in value and opposite in sign. This is the condition for optimum suppression of the stray field of the magnet coil system. Due to the great technical importance of actively shielded magnets, it is particularly advantageous that the effective field strength in the working volume gDeff of additional field-generating coil systems can also be increased for magnets of this type through the diamagnetic shielding behavior of the superconductor in the magnet coil system in accordance with the invention.
In another advantageous further development of these embodiments, the magnet coil system forms a first current path which is superconductingly short-circuited during operation, and a disturbance compensation coil which is galvanically not connected to the magnet coil system is disposed coaxially to the magnet coil system to form a further current path which is superconductingly short-circuited during operation. The disturbance compensation coil improves the temporal stability of the magnetic field in the working volume in response to external field fluctuations. In such a further development of an inventive magnet arrangement, the influence of a disturbance compensation coil on the effective field strength in the working volume gDeff of the additional field-generating coil system is taken into consideration.
In a further advantageous development, a part of the magnet coil system bridged with a superconducting switch forms a further current path which is superconductingly short-circuited during operation. An arrangement of this type improves the temporal stability of the magnetic field in the working volume in response to external field fluctuations. In such a further development of an inventive magnet arrangement the effect of bridging part of the magnet coil system with a superconducting switch on the effective field strength in the working volume gDeff of an additional field-generating coil system is taken into consideration.
In a further advantageous development of the inventive magnet arrangement, a system for compensating the drift of the magnet coil system forms a further current path which is superconductingly short-circuited during operation. Such an arrangement improves the temporal stability of the magnetic field in the working volume. In this further development of the inventive magnet arrangement, the influence of drift compensation on the effective field strength in the working volume gDeff of an additional field-generating coil system is taken into consideration.
In a further advantageous development, a shim device forms a further current path which is superconductly short-circuited during operation. Such an arrangement can compensate for field inhomogeneities. In this further development of the inventive magnet arrangement the influence of the superconducting shim device on the effective field strength gDeff of an additional field-generating coil system in the working volume is taken into consideration.
In a particularly preferred embodiment of the inventive magnet arrangement, a device having a radially inner and a radially outer partial coil forms a further current path which is superconductingly short-circuited during operation, wherein the partial coils are connected in series and the radially outer partial coil has a considerably higher dipole moment per ampere current than the radially inner partial coil, wherein the radially inner partial coil generates a considerably larger magnetic field per ampere current in the working volume than the radially outer partial coil. Such a device can increase the effective field strength in the working volume gDeff of an additional field-generating coil system if the additional field-generating coil system is disposed outside of the radially outer partial coil.
In a particularly advantageous further development of an inventive magnet arrangement, the additional field-generating coil system is normally conducting. In this arrangement, the additional field-generating coil system can advantageously be mounted in a room temperature region without influencing the cooling of the superconducting part of the magnet arrangement.
A further advantageous development of an inventive magnet arrangement is characterized in that the additional field-generating coil system is superconducting. In this arrangement, the current-carrying capacity of the additional field-generating coil system is advantageously larger than that of resistive coils.
In an advantageous further development of an inventive arrangement, the additional field-generating coil system is part of a device for modulating the magnetic field strength in the working volume. Dimensioning of such a coil system is particularly efficient in the inventive arrangement.
In a further advantageous development, the additional field-generating coil system is part of a so-called Z0 shim, generating a substantially homogeneous magnetic field in the working volume. A current change in such a device compensates for a drift of the main coil system after a certain period of time without having to reset the current in the main coil system. The inventive arrangement permits particularly efficient dimensioning of such a device.
The present invention also concerns a method for dimensioning an inventive magnet arrangement which is characterized in that the variable gDeff, which corresponds to the field change in the working volume at z=0 per ampere current in the additional field-generating coil system, is calculated taking into consideration the magnetic fields produced by the currents induced in the residual magnet arrangement according to:
xe2x80x83gDeff=gDxe2x88x92gTxc2x7(Lclxe2x88x92xcex1Lcor)xe2x88x921xc2x7(L←Dclxe2x88x92xcex1L←Dcor)
wherein the variables have the same, above-mentioned definitions. With this method for dimensioning a magnet arrangement having an additional field-generating coil system, the magnetic shielding behavior of the superconductor in the magnet coil system is advantageously taken into consideration. The method is based on the calculation of correction terms for the inductive couplings and for all self-inductances, which influence the respective quantities with a weighting factor xcex1. This method produces better agreement between calculated and measurable effective field strength in the working volume gDeff of the additional field-generating coil system than with a method according to prior art. The magnet arrangement can be optimized by making gDeff as large as possible while taking into account the magnetic shielding behavior of the superconductor in the magnet coil system.
In a simple variant of the inventive method, the parameter xcex1 corresponds to the volume portion of the superconducting material in the overall volume of the magnet coil system. This method for determining the parameter xcex1 is based on the assumption that the susceptibility in the superconductor with respect to small field changes is (xe2x88x921) (ideal diamagnetism).
The values for xcex1 determined in this fashion cannot be confirmed experimentally for most magnet types. Therefore, in a particularly preferred alternative method variant, the parameter xcex1 is determined experimentally for the magnet coil system from the measurement of the variable xcex2exp of the magnet coil system [without additional current paths which react inductively to flux changes] in response to a disturbance coil generating a substantially homogeneous disturbance field in the volume of the magnet coil system and through insertion of the variable xcex2exp into the equation       α    =                                        (                                          g                H                            ⁢                              (                                  L                  M                  cl                                )                                      )                    2                ⁢                  (                                    β              exp                        -                          β              cl                                )                                                                g              H                        ⁢                          (                                                β                  exp                                -                                  β                  cl                                            )                                ⁢                      L            M            cl                    ⁢                      L            M            cor                          -                              g            M                    ⁢                      (                                                            L                                      M                    ←                    H                                    cl                                ⁢                                  L                  M                  cor                                            -                                                L                                      M                    ←                    H                                    cor                                ⁢                                  L                  M                  cl                                                      )                                ,
wherein             β      exp        =                  g        H        exp                    g        H              ,
gHexp: measured field change in the working volume of the magnet arrangement per ampere current in the disturbance coil,             β      cl        =          1      -                        g          M                ·                  (                                    L                              M                ←                H                            cl                                                      L                M                cl                            ·                              g                H                                              )                      ,      xe2x80x83    ⁢  with
gM: Field per ampere of the magnet coil system in the working volume,
gH: Field per ampere of the disturbance coil in the working volume without the field contributions of the magnet coil system,
LMcl: Inductance of the magnet coil system,
LM←Hcl: Inductive coupling between the disturbance coil and the magnet coil system,
LMcor: Correction for the inductance LMcl of the magnet coil system, which would result with complete diamagnetic expulsion of disturbance fields from the volume of the magnet coil system,
LM←Hcor: Correction for the inductive coupling LM←Hcl of the disturbance coil with the magnet coil system which would result with complete diamagnetic expulsion of disturbance fields from the volume of the magnet coil system.
Finally, in a further particularly preferred variant of the inventive method, the corrections Lcor, L←Dcor, LMcor and LM←Dcor are calculated as follows:             L      cor        =          (                                                  L              M              cor                                                          L                              M                ←                P1                            cor                                            …                                              L                              M                ←                Pn                            cor                                                                          L                              P1                ←                M                            cor                                                          L              P1              cor                                            …                                              L                              P1                ←                Pn                            cor                                                            ⋮                                ⋮                                ⋰                                ⋮                                                              L                              Pn                ←                M                            cor                                                          L                              Pn                ←                P1                            cor                                            …                                              L              Pn              cor                                          )        ,            L              ←        D            cor        =          (                                                  L                              M                ←                D                            cor                                                                          L                              P1                ←                D                            cor                                                            ⋮                                                              L                              Pn                ←                D                            cor                                          )        ,xe2x80x83LPj←Pkcor=fPj(L(Pj,red,Ra1)←Pkclxe2x88x92L(Pj,red,Ri1)←Pkcl),
LPj←Dcor=fPj(L(Pj,red,Ra1)←Dclxe2x88x92L(Pj,red,Ri1)←Dcl), 
LPj←Mcor=fPj(L(Pj,red,Ra1)←Mclxe2x88x92L(Pj,red,Ri1)←Mcl),                                           L                          M              ←              Pj                        cor                    =                      xe2x80x83                    ⁢                                    L                              1                ←                Pj                            cl                        -                          L                                                (                                      1                    ,                    red                    ,                                          Ri                      1                                                        )                                ←                Pj                            cl                        +                                                            Ra                  1                                                  R                  2                                            ⁢                              (                                                      L                                                                  (                                                  2                          ,                          red                          ,                                                      Ra                            1                                                                          )                                            ←                      Pj                                        cl                                    -                                      L                                                                  (                                                  2                          ,                          red                          ,                                                      Ri                            1                                                                          )                                            ←                      Pj                                        cl                                                  )                                                    ,                                                      L                          M              ←              D                        cor                    =                      xe2x80x83                    ⁢                                    L                              1                ←                D                            cl                        -                          L                                                (                                      1                    ,                    red                    ,                    Ri1                                    )                                ←                D                            cl                        +                                                            Ra                  1                                                  R                  2                                            ⁢                              (                                                      L                                                                  (                                                  2                          ,                          red                          ,                                                      Ra                            1                                                                          )                                            ←                      D                                        cl                                    -                                      L                                                                  (                                                  2                          ,                          red                          ,                                                      Ri                            1                                                                          )                                            ←                      D                                        cl                                                  )                                                    ,                                          L          M          cor                =                  xe2x80x83                ⁢                              L                          1              ←              1                        cl                    -                      L                                          (                                  1                  ,                  red                  ,                  Ri1                                )                            ←              1                        cl                    +                      L                          1              ←              2                        cl                    -                      L                                          (                                  1                  ,                  red                  ,                  Ri1                                )                            ←              2                        cl                    +                                                  xe2x80x83                ⁢                                            Ra              1                                      R              2                                ⁢                      (                                          L                                                      (                                          2                      ,                      red                      ,                                              Ra                        1                                                              )                                    ←                  2                                cl                            -                              L                                                      (                                          2                      ,                      red                      ,                                              Ri                        1                                                              )                                    ←                  2                                cl                            +                              L                                                      (                                          2                      ,                      red                      ,                                              Ra                        1                                                              )                                    ←                  1                                cl                            -                              L                                                      (                                          2                      ,                      red                      ,                                              Ri                        1                                                              )                                    ←                  1                                cl                                      )                              
wherein
Ra1: Outer radius of the magnet coil system (in case of an actively shielded magnet coil system the outer radius of the main coil),
Ri1: Inner radius of the magnet coil system,
R2: in case of an actively shielded magnet coil system, the average radius of shielding, otherwise infinite,
RPj: average radius of the additional coil Pj,       f    Pj    =      {                                                                      Ra                1                                            R                Pj                                      ,                                          R                Pj                             greater than                               Ra                1                                                                                      1            ,                                          R                Pj                             less than                               Ra                1                                                        
and wherein the index 1 corresponds to the main coil for an actively shielded magnet coil system and otherwise represents the magnet coil system. The index 2 signifies the shielding for an actively shielded magnet coil system which in the absence thereof, is omitted. The index (X, red, R) designates a hypothetical coil X all of whose windings are located at radius R.
The particular advantage of this method for calculating the corrections Lcor, L←Dcor, LMcor and LM←Dcor consists in that the corrections are derived using the inductive couplings and self-inductances of coils and taking into consideration the geometric arrangement of the coils concerned.
Further advantages of the invention can be extracted from the description and the drawing. The features mentioned above and below can be used in accordance with the invention either individually or collectively in arbitrary combination. The embodiments shown and described are not to be considered to be exhaustive enumeration, but rather have exemplary character for describing the invention.
The invention is shown in the drawing and explained in more detail with reference to embodiments.